The invention relates to marine propulsion systems with automatic oil-fuel mixing for a two cycle crankcase compression internal combustion engine.
Marine propulsion systems with automatic oil-fuel mixing is shown in commonly owned copending U.S. application Ser. No. 07/182,180, filed Apr. 15, 1988, entitled "Marine Engine With Combination Vapor Return, Crankcase Pressure, And Cooled Fuel Line Conduit", still pending, U.S. Pat. No. 4,583,500, entitled "Marine Propulsion System With Automatic Oil-Fuel Mixing", incorporated herein by reference. The mixing valve automatically mixes oil and fuel, and eliminates the need to premix same. The valve has a fuel inlet, an oil inlet, and an oil-fuel outlet, and is operated by a pressure differential between the fuel inlet and the oil-fuel outlet across a diaphragm as shown at 24 in FIG. 2 and at 324 in FIG. 6 of U.S. Pat. No. 4,583,500. The mixing valve may be connected downstream of the fuel pump and use the fuel pressure to operate the mixing valve. Alternatively, the mixing valve may be connected upstream of the fuel pump and use the fuel pump suction to operate the mixing valve. In other applications where a pressurized fuel tank is utilized, such pressure may be used to operate the valve. In other implementations, crankcase pressure and/or vacuum may be used to operate the valve, or a separate dedicated small pump may be used, all as noted in U.S. Pat. No. 4,583,500.
The present invention arose during development efforts directed toward reducing the size of the oil-fuel mixing components and system to enable such system to fit inside the engine cowl of an outboard two cycle internal combustion engine. The invention provides a dual fuel pump arrangement accomplishing such objective.
The oil-fuel mixing valve was previously external to the engine cowl. The external mixing valve was provided with a large diaphragm to operate the mechanism and pump oil in response to fuel pressure differential. The diaphragm was made as large as possible to reduce the pressure drop across the diaphragm between the fuel inlet and the oil-fuel outlet of the mixing valve. This was necessary in implementations where the mixing valve is connected upstream of the fuel pump, and the fuel is drawn or pulled through the mixing valve by fuel pump suction. The pressure drop across the diaphragm must be as low as possible because fuel will otherwise vaporize in a vacuum.
In order to reduce the size of the mixing valve to enable it to fit within the engine cowl, a smaller diaphragm or piston must be used. This increases the pressure drop across the diaphragm, and in turn requires that the mixing valve be connected downstream of the fuel pump so that pressurized fuel is pushed into the mixing valve. This minimizes the fuel vaporization problem. However, to accomplish such arrangement economically and in a small package was found to be very difficult.
In various marine propulsion systems, such as those including a Mercury Marine 25 horsepower outboard two cycle engine, a crankcase pressure driven fuel pump is provided which is integral with the carburetor Mercury Marine Brunswick Corp. Quicksilver Parts Catalog 20 H.P. 25 H.P., 90-18583, September 1987, page 14-15. This is more economical than mounting the fuel pump on the engine block crankcase and running hoses to the carburetor, as is done on larger engines. However, this arrangement is not suitable for the above noted mixing application with a smaller mixing valve because the mixing valve would be upstream of the fuel pump.
The present invention provides the above noted mixing application and enables a reduced size mixing valve to fit within the engine cowl without sacrificing the economical mounting arrangement noted above for Mercury Marine 25 HP applications and the like if desired. The invention enables the use of known relatively small and inexpensive crankcase pulse pressure driven fuel pumps. A first fuel pump draws fuel from a remote fuel tank and supplies pressurized fuel to the inlet of the mixing valve. This minimizes the above noted vaporization problem. This first fuel pump by itself is not sufficient to supply the requirements of the engine. A second fuel pump draws oil-fuel from the outlet of the mixing valve and supplies same to the engine. The fuel pumps may be integral with the mixing valve and the carburetor, respectively, or may be separate.
In the preferred embodiment, the fuel pumps receive crankcase pressure pulses from crankcase chambers whose cylinders are 180.degree. out of phase with each other. In this manner, while the first pump is pushing pressurized fuel to the inlet of the mixing valve, the second pump is pulling suctioned oil-fuel from the outlet of the mixing valve. This increases overall pressure and capacity.
The system in accordance with the invention offers superior performance, lower cost, and smaller package size than previous single pump arrangements.